Nonlinear dynamics of radiative condensations in optically thin plasmas

Plasmas that cool by radiation may become unstable to the formation of localized regions of lower temperature and increased density. Such radiative condensations are common in astrophysical and laboratory plasmas that are optically thin. Some perspective of the rapidly developing theoretical understanding of the dynamics of radiative condensations can be given in the framework of simple models. Radiative condensations are closely related to the condensation mode of the thermal instability, first studied by G. B. Field. Progress in the analysis of the nonlinear stage of this instability, achieved recently, employs the hierarchy of time and length scales of the problem. Sets of reduced equations, which describe the nonlinear dynamics of the radiative condensation, have been developed separately in the long-, intermediate-, and short-wavelength limits. Being quite different, all these reduced models predict radiation-driven segregation of the unstable plasma into two different states ("phases") on an intermediate time scale. Subsequent long-time evolution of radiative condensation may strongly depend on the type of boundary condition for the plasma. Radiative condensations usually disappear on a longer (thermal conduction related) time scale, if an inflow/outflow of the plasma is allowed. In this case, regions occupied by one of the "phases" expand until they occupy the whole plasma. On the contrary, in confined plasmas (no inflow/outflow of the plasma is allowed), radiative condensations can persist "forever." Similarities have been explored between radiative condensations and a number of instabilities of growth, such as the Darrieus-Landau instability and Ostwald ripening. A new type of shock wave with a nonmonotonic pressure profile, resulting from radiative condensation dynamics, is described. The role of magnetic fields in radiative condensations is briefly discussed.